The 20-year-old, who moved to Upton Park from Old Trafford in January 2012, has made an impressive start to the campaign and received his first England Under-21s cap in the 4-0 win over San Marino on Thursday.

The West Ham boss is unsure whether the Premier League champions have the option to re-sign Morrison and concedes that his side could struggle to keep the midfielder if a substantial offer is received.

"I don't know if there is a buy-back clause and what that figure might be if it even exists," Allardyce told the press.

"But my real big worry is that Manchester United will come back to buy him.

"Look at what happened when Chris Smalling played a handful of games for Fulham - United jumped in with £10 million. Then there was Phil Jones when I was at Blackburn – that's what you are up against."